The present invention relates generally liquid crystal compounds and compositions and to optical devices employing liquid crystal compositions in optical switching and display elements. The invention more specifically relates to antiferroelectric liquid crystal compositions and improved liquid crystal compositions that exhibit V-shaped switching and bistable switching exhibiting very fast switching speeds and wide view angles useful in the preparation of LC optical devices. The invention also relates to analog and bistable LC optical devices that employ these compositions and which exhibit very fast switching speeds and wide view angles.
Several types of smectic liquid crystal materials (LCs) have been investigated for rapid switching, view-angle enhancement and higher contrast, including surface-stabilized ferroelectric LCs (FLCs), deformed helix ferroelectric LCs (DHFLCs), and antiferroelectric LCs (AFLCs). Recently, smectic material exhibiting thresholdless or more properly V-shaped switching LCs (VLCs) have been described (Inui, S. et al. (1996) J. Mater. Chem. 6(4):671–673; Seomun, S. S. et al. (1997) Jpn. J. Appl. Phys. 36:3580–3590).
Liquid crystal (LC) compositions exhibit one or more LC phases. LC compositions may be composed of one or more components. Components of LC compositions may exhibit liquid crystal phases, have latent liquid crystal phases or be compatible with (not suppress) liquid crystal phases in the LC composition. LC compounds and components of LC mixtures of this invention are rod-like molecules most typically having a generally linear mesogenic core with one or more directly or indirectly linked alicyclic or aromatic rings (which may be fused aromatic rings) and linear or branched tail groups distributed on either side of the mesogenic core, e.g.:
LC components which do not themselves exhibit liquid crystal phases, but which exhibit LC phases on combination with one or more other components are described as having “latent” liquid crystal phases. Chiral nonracemic LCs useful in FLC, DHFLC, AFLC and VLC compositions have at least one component that has a chiral non-racemic tail group. FLC, DHFLC, AFLC and VLC compositions may be composed entirely of chiral non-racemic components, but are typically composed of a mixture of chiral nonracemic and achiral or racemic components.
Ferroelectric LCs when aligned parallel to the substrate surfaces using the surface stabilized effect (in an surface-stabilized ferroelectric liquid crystal (SSFLC) device) exhibit two stable state switching on a microsecond time scale. Antiferroelectric LCs exhibit three stable-state switching, which by application of a bias field can be converted for use in a bistable switching mode LC devices. Two of the AFLC states have the same transmittance, so that alternate symmetrical switching can be used in AFLC devices. VLCs, in contrast, exhibit very rapid, analog electro-optic response, allow symmetrical driving, and no dc balance is required. VLCs are particularly attractive for applications requiring generation of multiple levels of gray scale.
High quality full color images in a flat panel display requires at least sixteen levels of gray scale. Temporal gray scale or partial domain switching techniques have been used to adapt bistable state FLC devices to multiple level gray scale applications. However, the number of gray levels that can be generated with such methods is limited. An electroclinic effect, that has been observed in certain chiral nonracemic LC compositions possessing a smectic A phase, has also been employed to generated multiple gray scale levels. In an electroclinic LC device, application of an electric field to the LC in the chiral smectic A phases induces the LC molecules to tilt. The tilt angle is linearly proportional to the applied electric field and results in the generation of analog gray scale. The induced tilt angle is, however, temperature dependent and the maximum tilt angle available in most electroclinic compositions is small requiring temperature control and limiting device contrast. In contrast, LC compositions exhibiting V-shaped switching can exhibit large tilt angles that are insensitive to temperature, and have minimal hysterisis and minimal LC defects. These properties allow construction of LC devices with very fast response time, large viewing angle and high contrast.
V-shaped LC compositions and components of such compositions are useful in a variety of optical device applications, including, in particular, active matrix and thin film transistor display applications (e.g., flat-panel displays, computer monitors, head-mounted displays, cellular phone viewers), in analog beam deflectors (which can, e.g., replace spinning mirrors in bar code scanners), for optical correlation, and for on-the-fly adaptive optics (for use, e.g., in astronomy and robotic vision).
The thresholdless effect was first observed by Inui et al. (1996) J. Mater. Chem. 6:671 in a three component antiferroelectric LC mixture of compounds A:B:C (40:40:20 mass %):

It was later reported by Seong et al. (1997) J. Appl. Phys. 36:3586–3590 that compound A in this mixture when homogeneously aligned in an LC cell exhibited V-shaped switching in an antiferroelectric phase at certain temperatures.
Several models have been proposed to explain V-shaped switching. In one model, based on the association of V-shaped switching with LCs having antiferroelectric phases, a distinct type of antiferroelectric phase is proposed to be the origin of V-shaped switching (Matsumoto T., et al. (1999) J. Mater. Chem. 9:2051–2080). In this phase, the LC molecules within a layer are uniformly tilted, but the tilt direction of each layer is randomly distributed, rather than layer correlated as in the ordinary antiferroelectric phase. The analog electrooptic response results when LC molecules having different relative tilt orientation respond differently to an applied field. As the field is increased more and more molecules align with the applied field until the LC material reaches the ferroelectric state. The randomly tilted phase was designated a thresholdless antiferroelectric liquid crystal (TAFLC) phase.
In another model, the V-shaped switching phase is described as a chevron-type smectic C phase (Parl, B. et al. (1999) Physical Review 59(4):R3815). In this model at zero field, the LC molecules are uniformly aligned along the chevron interface. When an electric field is applied the LC molecules rotate back and forth along a half cone about their aligned orientation. Molecules rotate differently above and below the chevron interface resulting in an analog electrooptic response with increasing field.
A third model is based on an experimental determination that V-shaped switching can occur in a randomly tilted smectic C phase which is aligned in a bookshelf layer structure between parallel substrates (Two Clarke et al. references). In this model, V-shaped switching depends upon the ability of the LC to form a bookshelf layer structure and high spontaneous polarization (Ps) of the LC. In the bookshelf geometry with LC molecules exhibiting high Ps, the polarization orients as a uniform block. When an electric field is applied the uniform polarization block responds by azimuthal orientation of Ps on the tilt cone. Bookshelf geometry has previously been described in a class of naphthalene type LCs (U.S. Pat. No. 5,348,685 and Mochizuki et al. (1991) Ferroelectrics 122:37–51). A bookshelf layer structure can be formed in the smectic C phase when there is little or no shrinkage of the layer spacing on transition from the smectic A phase to the smectic C phase. Materials which exhibit a “deVries” type smectic A phase will form bookshelf layer geometry (U.S. provisional application, 60/151,974, filed Sep. 1, 1999, U.S. application Ser. No. 09/653,437, filed Sep. 1, 2000 which is incorporated by reference in its entirety herein). A de Vries smectic A phase (the existence of which was first suggested by de Vries, A. (1977) Mol. Cryst. Liq. Cryst. 41:27 and de Vries, A. (1979) Mol. Cryst. Liq. Cryst. 4:179) consists of LC molecules whose directors are tilted with respect to the layer normal (rather than parallel to the layer normal in a regular smectic A phase). However, the titled LC molecules are randomly oriented with respect to each other such that the average director of the LC is parallel to the layer normal. There is little shrinkage on transition from a de Vries smectic A to a smectic C because the LC molecules are already tilted. This third model suggests that V-shaped switching will be associated with LC molecules which exhibit a de Vries smectic A phase and possess high Ps.
U.S. Pat. No. 6,045,720 relates to LC compounds having the chiral tail:
where R1 is CH3, CF3, CH2F, CHF2, m is 2–12 and R2 is an alkyl group having 1–10 carbon atoms with three-ring mesogenic cores which exhibit an antiferroelectric phase. The mesogenic core can contain phenyl, various F-substituted phenyl, pyridine rings and cyclohexy rings. The achiral tail is an alkyl or alkoxy group. Certain compounds of the invention are reported to exhibit low threshold switching.
Several models have been proposed to explain V-shaped switching. In one model, based on the association of V-shaped switching with LCs having antiferroelectric phases, a distinct type of antiferroelectric phase is proposed to be the origin of V-shaped switching.
U.S. Pat. No. 5,938,973 relates to certain ferrielectric LC compositions containing certain swallow-tailed LC compounds. The composition also contains certain chiral LC compounds having a chiral tail group of Formula D:
where A is CF3 and B is certain ether groups. The mesogenic core is a phenyl benzoate core which may be substituted with one fluorine and the other tail group is an alkoxy group. The reference reports an apparently continuous change in transmission in on application of voltage between 0 and 4 volts. It is suggested that the continuous change in transmission as a function of voltage results because the threshold voltage between ferroelectric and antiferroelectric states in the LC composition is “not distinct.”
U.S. Pat. No. 5,728,864 relates to LC compounds or compositions having a ferrielectric phase. The LC compounds have a biphenyl benzoate core optionally substituted with one fluorine with a chiral tail of formula D where A is CF3 or C2F5 and B is certain ether and an achiral tail that is a linear alkoxy group.
U.S. Pat. No. 6,002,042 relates to compounds having an antiferroelectric or a ferrielectric phase that are biphenyl benzoates optionally substituted with one fluorine with a chiral tail group of formula D where A is CF3 and B is certain alkyl groups and the other tail group is a linear alkoxy group.
A number of additional compounds having biphenyl benzoate cores and the same or a similar chiral or achiral tail group D (above) where A is —CF3, —CH3, —C2H5, and B is various alkyl or ether groups and when the compound is chiral * indicates the chiral carbon. U.S. Pat. No. 5,340,498 relates to compounds having certain fluorine substitution on the phenyl benzoate core and ether groups.
U.S. Pat. Nos. 5,980780 and 5,985,172 relate to antiferroelectric LC compositions containing certain racemic phenyl benzoate compounds in combination with compounds having a biphenyl benzoate core optionally substituted with one fluorine and having a chiral tail of formula D where A is —CF3 or —CH3 and B is certain alkyl or ether tails.
U.S. Pat. No. 6,018,070 relates to antiferroelectric LC compounds (which may be optically active) having a two-ring phenyl ester core which may have certain fluorine substituents on the ring where one tail is an alkyl ester and the other tail is the tail of formula D where A is —CH3 or —CF3 and B can be certain alkyl or ether groups.
U.S. Pat. No. 6,001,278 relates to certain antiferroelectric LC compositions containing certain swallow-tailed LC compounds. The composition also contains certain antiferroelectric LC compounds having a chiral tail group of Formula D where A is CH3 or CF3 and B is certain alkyl or ether groups. The mesogenic core is a phenyl benzoate core which may be substituted with one fluorine and the other tail is an alkyl ester tail.